Flow sensors are utilized in a variety of fluid-sensing applications for detecting the movement of fluids, particularly fluids in liquid form. One type of flow measurement, for example, is based on thermal sensors, which can also be utilized to detect various properties of a fluid. Thermal sensors may be implemented, for example, over a silicon substrate in microstructure form. For convenience sake, and without limitation, the term “flow sensor” can be utilized to refer to such thermal sensors. (See e.g. U.S. Pat. No. 6,322,247 FIGS. 10a-f, and U.S. Pat. No. 6,184,773, which are both incorporated herein by reference.). The reader will appreciate that such sensors may also be utilized to measure intrinsic fluid properties such as thermal conductivity, specific heat (e.g. U.S. Pat. Nos. 5,237,523 and 5,311,447, which are both incorporated herein by reference.), non-intrinsic properties such as temperature, flow velocity, flow rate, and pressure, and other properties; and that the flows may be generated through forced or natural convection.
A thermal-type flow sensor can be formed from a substrate that includes a heating element and one or more heat-receiving, or sensing, elements. If two such sensing elements are utilized, they can be positioned at the upstream and downstream sides of the heating element relative to the direction of the fluid flow to be measured. When fluid flows along the substrate, it is heated by the heating element at the upstream side and the heat is then transferred non-symmetrically to the heat-receiving elements on either side of the heating element. Since the level of non-symmetry depends on the rate of fluid flow, and that non-symmetry can be sensed electronically, such a flow sensor can be used to determine the rate and the cumulative amount of the fluid flow.
Such flow sensors generally face potential problems when the sensors are used in a tilted, or non-horizontal orientation. Heat usually causes the density of a fluid to change. Less dense fluid tends to rise, while the more dense fluid falls. This circulation, or natural convection, generates a signal in a vertically oriented flow sensor, but generates no signal in a horizontally oriented flow sensor. The natural convection is added to the forced convection flow generated by a pump, causing discrepancies in the measured properties, specifically flow rate, of a flow sensor in a non-horizontal orientation. The tilt angle of the flow sensor is one of the factors influencing the natural convection flow in a flow sensor.
The usual solution to avoid such interference or discrepancies in measured properties is to mount the flow sensor in a horizontal position to avoid any tilting. Another solution described in U.S. Pat. No. 4,100,801, is to place the thermal flow sensor in a by-pass loop coupled to the fluid conduit. The present invention provides a flow sensor that is insensitive to tilting without diverting the fluid flow.